This invention relates to a relay arrangement for interrupting and switching the flow of alternating current, especially from a three phase alternating current power source to a three phase load, and means for controlling the relay arrangement. The present invention can also be applied to single phase and other power transmission systems.
Three phase alternating current power sources are commonly used on aircraft. These sources usually operate at a nominal voltage of 110-120 VRMS (155-170 v. peak) and a frequency of 400 Hz.; with the power being supplied by inverters coupled to engine-driven DC generators, or by engine-operated alternators. The aircraft usually also provide battery power at a nominal voltage of 28 v.d.c.
Circuit breakers are used on the aircraft to disconnect the power sources from various loads under overcurrent and/or (in the case of three phase power sources) phase imbalance conditions.
It is desirable that the mechanism which determines when to interrupt the flow of current be capable of responding to overcurrent or phase imbalance conditions according to a predetermined time-dependent function. That is, for small overloads there should be a relatively long response time before current flow is interrupted, so as to avoid tripping in response to temporary overloads due, for example, to motor startups; while for large overloads the response should be rapid so as to prevent damage to equipment, power sources or wiring. It is also desirable that the current flow interrupting device be remotely resettable and capable of interrupting current flow in response to a remote command, so that the device also functions as a power control switch.
In an aircraft it is necessary to provide all these features while minimizing the size and weight of the power controller, especially since a modern aircraft normally requires many of such devices.
Accordingly, it is an object of the present invention to provide an improved power controller which is especially suitable for, but not limited to, use on aircraft.